Wells in unconsolidated formations such as those found in the Gulf of Mexico, are known to have certain sand production problems. For example, the fact that the formations are unconsolidated requires that each well is "cased" with a tubular steel casing and that the casing be then sealed to the formation by pumping cement down around the casing. Thereafter, the well is "perforated" at selected depths, typically by using an explosive charge and projectile to create perforations through both the steel casing and the cement into the formation wherein hydrocarbons are believed to be present. Washing of the perforation by pumping of a completion fluid, such as a "weighted" brine having a higher specific gravity than the fluids contained in the formation, down a work string, and through a perforation wash tool and finally out through the perforations removes debris from the areas of the perforations and may create a void in the unconsolidated sand-like formation behind the cement sheath. It is then common practice to pump "gravel" slurries down the hole and out through the perforations to fill the perforations and any such voids. ("Gravel" is used here, as usual in the art, to refer to a more coarse sand than that typically found in the formation.) The gravel then fills the perforations and any voids and restricts the movement of the finer formation sand behind the cement and the casing.
It is necessary to know what quantity of voids are to be filled by the gravel in order to evaluate the amount of gravel required. In order that the amount of void space to be filled by gravel can be known, the amount of voids created by the perforation washing with brine also must be known. There is at present no completely satisfactory technique for measuring the amount and size of these voids.
It is accordingly an object of the invention to provide a method for evaluating the quantity and size of voids formed during perforation washing.
A broad object of the invention is to provide a means for better evaluating the behind-casing configuration of void space.
Still another object of the invention is to provide improved methods of well completion, whereby an improved well will result.